1. Field of the Invention
The present invention relates to an amplifier circuit capable of improving phase characteristics, and a voltage regulator including the amplifier circuit.
2. Description of the Related Art
FIG. 5 is a circuit diagram of a related-art negative feedback amplifier circuit.
A related-art negative feedback amplifier circuit 500 includes an amplifier circuit 510, which is a common source amplifier circuit, and an amplifier circuit 520. The amplifier circuit 510 includes a current source 511 and an NMOS transistor 512, which are connected in series.
The amplifier circuit 510 has an output connected to an input of the amplifier circuit 520. The amplifier circuit 520 has an output connected to a gate of the NMOS transistor 512 of the amplifier circuit 510.
The amplifier circuit 510 is configured to amplify, based on a drive current of the NMOS transistor 512, a voltage input to the amplifier circuit 510 and to output the amplified voltage. The amplifier circuit 520 is configured to amplify an output voltage V1a of the amplifier circuit 510 and to output the amplified voltage. A feedback voltage V2 generated by the amplifier circuit 520 is input to the amplifier circuit 510.
Therefore, the negative feedback amplifier circuit 500 is configured to operate so as to maintain an operating point at a constant value. For example, the output voltage V1a of the amplifier circuit 510 and the output voltage V2 of the amplifier circuit 520 may be maintained at a constant value by causing the drive current of the NMOS transistor 512 to be approximately equal to a current of the current source 511 (see, for example, Japanese Patent Application Laid-open No. Hei 7-183736).
However, in the related-art negative feedback amplifier circuit 500, there is a problem in that a phase of a feedback voltage is delayed due to a pole appearing in output of an amplifier circuit, and that a negative feedback circuit thus has a high risk of oscillating.
A frequency at which the pole appears is affected by a load capacitance and a load resistance. For example, when the load capacitance is small and the load resistance is small, the pole appears at a high frequency and causes a phase delay. Further, for example, when the load capacitance is large and the load resistance is large, the pole appears at a low frequency and causes a phase delay. Here, conditions of the load capacitance or the load resistance differ depending on an application.
In order for the negative feedback circuit to have a lower risk of oscillating, it is important to correctly know the frequency at which the pole appears and take measures based on the conditions of the load capacitance or the load resistance of the application to be faced.